This invention relates to a process for converting methanol to light olefins with increased selectivity to ethylene and propylene. The process comprises contacting the methanol with a catalyst comprising a metallo aluminophosphate molecular sieve having an empirical formula of (ELxAlyPz)O2 where EL includes silicon and characterized in that xe2x80x9cxxe2x80x9d has a value from about 0.02 to about 0.08. A preferred catalyst is one where the molecular sieve has predominantly a plate crystal morphology such that the average smallest crystal dimension is at least 0.1 micron and has an aspect ratio of less than or equal to 5.
The limited supply and increasing cost of crude oil has prompted the search for alternative processes for producing hydrocarbon products. One such process is the conversion of methanol to hydrocarbons and especially light olefins (by light olefins is meant C2 to C4 olefins). The interest in the methanol to olefin (MTO) process is based on the fact that methanol can be obtained from coal or natural gas by the production of synthesis gas which is then processed to produce methanol.
Processes for converting methanol to light olefins are well known in the art. Initially aluminosilicates or zeolites were used as the catalysts necessary to carry out the conversion. For example, see U.S. Pat. No. 4,238,631 B1; U.S. Pat. No. 4,328,384 B1, U.S. Pat. No. 4,423,274 B1. These patents further disclose the deposition of coke onto the zeolites in order to increase selectivity to light olefins and minimize the formation of C5+ byproducts. The effect of the coke is to reduce the pore diameter of the zeolite.
The prior art also discloses that silico aluminophosphates (SAPOs) can be used to catalyze the methanol to olefin process. Thus, U.S. Pat. No. 4,499,327 B1 discloses that many of the SAPO family of molecular sieves can be used to convert methanol to olefins. The ""327 patent also discloses that preferred SAPOs are those that have pores large enough to adsorb xenon (kinetic diameter of 4.0 xc3x85) but small enough to exclude isobutane (kinetic diameter of 5.0 xc3x85). A particularly preferred SAPO is SAPO-34.
U.S. Pat. No. 4,752,651 B1 discloses the use of nonzeolitic molecular sieves (NZMS) including ELAPOs and MeAPO molecular sieves to catalyze the methanol to olefin reaction.
The effect of the particle size of the molecular sieve on activity has also been documented in U.S. Pat. No. 5,126,308 B 1. In the ""308 patent it is disclosed that molecular sieves in which 50% of the molecular sieve particles have a particle size less than 1.0 xcexcm and no more than 10% of the particles have a particle size greater than 2.0 xcexcm have increased activity and/or durability. The ""308 patent also discloses that restricting the silicon content to about 0.005 to about 0.05 mole fraction also improves catalyst life by reducing coke formation.
In contrast to this art, applicants have found that a methanol to olefin process using molecular sieves having the empirical formula (ELxAlyPz)O2 (hereinafter ELAPO) where EL is a metal selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures thereof and xe2x80x9cxxe2x80x9d, xe2x80x9cyxe2x80x9d and xe2x80x9czxe2x80x9d are the mole fractions of EL, Al and P respectively and specifically where xe2x80x9cxxe2x80x9d has a value from about 0.02 to about 0.08 provides an increased selectivity to ethylene and propylene with a reduction in undesirable C4s and C5+. A preferred catalyst is one which additionally has a predominantly plate crystal morphology wherein the average smallest crystal dimension is at least 0.1 micron and has an aspect ratio of less than or equal to 5
As stated, this invention relates to a process for converting methanol to light olefins using a catalyst comprising an ELAPO molecular sieve. Accordingly, one embodiment of the invention is a process for converting methanol to light olefins comprising contacting the methanol with a catalyst at conversion conditions to provide the olefins, the catalyst comprising a crystalline metallo aluminophosphate molecular sieve having a chemical composition on an anhydrous basis expressed by an empirical formula of:
(ELxAlyPz)O2
where EL is a metal selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures thereof, xe2x80x9cxxe2x80x9d is the mole fraction of EL and has a value of about 0.02 to about 0.08, xe2x80x9cyxe2x80x9d is the mole fraction of Al and has a value of at least 0.01, xe2x80x9czxe2x80x9d is the mole fraction of P and has a value of at least 0.01 and x+y+z=1.
In another embodiment, the molecular sieve is characterized in that it has a crystal morphology wherein the average smallest crystal is at least 0.1 micron and has an aspect ratio no greater than 5.
These and other objects and embodiments of the invention will become more apparent after the detailed description of the invention.
An essential feature of the process of the instant invention is an ELAPO molecular sieve. ELAPOs are molecular sieves which have a three-dimensional microporous framework structure of AlO2, PO2 and ELO2 tetrahedral units. Generally the ELAPOs have the empirical formula:
(ELxAlyPz)O2
where EL is a metal selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures thereof, xe2x80x9cxxe2x80x9d is the mole fraction of EL and has a value of at least 0.005, xe2x80x9cyxe2x80x9d is the mole fraction of Al and has a value of at least 0.01, xe2x80x9czxe2x80x9d is the mole fraction of P and has a value of at least 0.01 and x+y+z=1. When EL is a mixture of metals, xe2x80x9cxxe2x80x9d represents the total amount of the metal mixture present. Preferred metals (EL) are silicon, magnesium and cobalt with silicon being especially preferred. As will be shown in the examples, when xe2x80x9cxxe2x80x9d has a value of about 0.02 to about 0.08 a greater selectivity to ethylene and propylene with a diminished selectivity to C4+ components is observed.
The preparation of various ELAPOs are well known in the art and may be found in U.S. Pat. No. 4,554,143 B1 (FeAPO); U.S. Pat. No. 4,440,871 B1 (SAPO); U.S. Pat. No. 4,853,197 B1 (MAPO, MnAPO, ZnAPO, COAPO); U.S. Pat. No. 4,793,984 B1 (CAPO), U.S. Pat. No. 4,752,651 B1 and U.S. Pat. No. 4,310,440 B1, all of which are incorporated by reference. Generally, the ELAPO molecular sieves are synthesized by hydrothermal crystallization from a reaction mixture containing reactive sources of EL, aluminum, phosphorus and a templating agent. Reactive sources of EL are the metal salts such as the chloride and nitrate salts. When EL is silicon a preferred source is fumed, colloidal or precipitated silica. Preferred reactive sources of aluminum and phosphorus are pseudo-boehmite alumina and phosphoric acid. Preferred templating agents are amines and quaternary ammonium compounds. An especially preferred templating agent is tetraethylammonium hydroxide (TEAOH).
The reaction mixture is placed in a sealed pressure vessel, optionally lined with an inert plastic material such as polytetrafluoroethylene and heated preferably under autogenous pressure at a temperature between about 50xc2x0 C. and 250xc2x0 C. and preferably between about 100xc2x0 C. and 200xc2x0 C. for a time sufficient to produce crystals of the ELAPO molecular sieve. Typically the time varies from about 1 hour to about 120 hours and preferably from about 24 hours to about 48 hours. The desired product is recovered by any convenient method such as centrifugation or filtration.
The ELAPO molecular sieves of this invention have predominantly a plate crystal morphology. By predominantly is meant greater than 50% of the crystals. Preferably at least 70% of the crystals have a plate morphology and most preferably at least 90% of the crystals have a plate morphology. Especially good selectivity (C2= versus C3=) is obtained when at least 95% of the crystals have a plate morphology. By plate morphology is meant that the crystals have the appearance of rectangular slabs. More importantly, the aspect ratio is less than or equal to 5. The aspect ratio is defined as the ratio of the largest crystalline dimension divided by the smallest crystalline dimension. A preferred morphology which is encompassed within the definition of plate is cubic morphology. By cubic is meant not only crystals in which all the dimensions are the same, but also those in which the aspect ratio is less than or equal to 2. It is also necessary that the average smallest crystal dimension be at least 0.1 microns and preferably at least 0.2 microns.
As is shown in the examples, the morphology of the crystals and the average smallest crystal dimension is determined by examining the ELAPO molecular sieve using Scanning Electron Microscopy (SEM) and measuring the crystals in order to obtain an average value for the smallest dimension.
Without wishing to be bound by any one particular theory, it appears that a minimum thickness is required so that the diffusion path for the desorption of ethylene and propylene is sufficiently long to allow differentiation of the two molecules. Since ethylene is a more valuable product, by controlling the crystal dimensions one can maximize the formation of ethylene. As will be shown in the examples, when the smallest dimension is less than 0.1, the ratio of ethylene to propylene (C2=/C3=) is about 1.2, whereas when the smallest dimension is greater than 0.1 microns, the ratio of C2=/C3= is about 1.4. This provides a greater production of ethylene.
The ELAPOs which are synthesized using the process described above will usually contain some of the organic templating agent in its pores. In order for the ELAPOs to be active catalysts, the templating agent in the pores must be removed by heating the ELAPO powder in an oxygen containing atmosphere at a temperature of about 200xc2x0 to about 700xc2x0 C. until the template is removed, usually a few hours.
As stated above unexpected selectivity is obtained when the metal (EL) content varies from about 0.02 to about 0.08 mole fraction, preferably from about 0.02 to about 0.07 and more preferably from about 0.03 to about 0.068. If EL is more than one metal then the total concentration of all the metals is between about 0.02 and 0.08 mole fraction. An especially preferred embodiment is one in which EL is silicon (usually referred to as SAPO). The SAPOs which can be used in the instant invention are any of those described in U.S. Pat. No. 4,440,871 B1. Of the specific crystallographic structures described in the ""871 patent, the SAPO-34, i.e., structure type 34, is preferred. The SAPO-34 structure is characterized in that it adsorbs xenon but does not adsorb isobutane, indicating that it has a pore opening of about 4.2 xc3x85.
The ELAPO molecular sieve of this invention may be used alone or they may be mixed with a binder and formed into shapes such as extrudates, pills, spheres, etc. Any inorganic oxide well known in the art may be used as a binder. Examples of the binders which can be used include alumina, silica, aluminum-phosphate, silica-alumina, etc. When a binder is used, the amount of ELAPO which is contained in the final product ranges from 10 to 90 weight percent and preferably from 30 to 70 weight percent.
The conversion of methanol to light olefins is effected by contacting the methanol with the ELAPO catalyst at conversion conditions, thereby forming the desired light olefins. The methanol can be in the liquid or vapor phase with the vapor phase being preferred. Contacting the methanol with the ELAPO catalyst can be done in a continuous mode or a batch mode with a continuous mode being preferred. The amount of time that the methanol is in contact with the ELAPO catalyst must be sufficient to convert the methanol to the desired light olefin products. When the process is carried out in a batch process, the contact time varies from about 0.001 hr. to about 1 hr. and preferably from about 0.01 hr. to about 1.0 hr. The longer contact times are used at lower temperatures while shorter times are used at higher temperatures. Further, when the process is carried out in a continuous mode, the Weight Hourly Space Velocity (WHSV) based on methanol can vary from about 1 hrxe2x88x921 to about 1000 hrxe2x88x921 and preferably from about 1 hrxe2x88x921 to about 100 hrxe2x88x921.
Generally, the process must be carried out at elevated temperatures in order to form light olefins at a fast enough rate. Thus, the process should be carried out at a temperature of about 300xc2x0 C. to about 600xc2x0 C., preferably from about 400xc2x0 C. to about 550xc2x0 C. The process may be carried out over a wide range of pressure including autogenous pressure. Thus, the pressure can vary from about 0 kPa (0 psig) to about 1724 kPa (250 psig) and preferably from about 34 kPa (5 psig) to about 345 kPa (50 psig).
Optionally, the methanol feedstock may be diluted with an inert diluent in order to more efficiently convert the methanol to olefins. Examples of the diluents which may be used are helium, argon, nitrogen, carbon monoxide, carbon dioxide, hydrogen, steam, paraffinic hydrocarbons, e.g., methane, aromatic hydrocarbons, e.g., benzene, toluene and mixtures thereof. The amount of diluent used can vary considerably and is usually from about 5 to about 90 mole percent of the feedstock and preferably from about 25 to about 75 mole percent.
The actual configuration of the reaction zone may be any well known catalyst reaction apparatus known in the art. Thus, a single reaction zone or a number of zones arranged in series or parallel may be used. In such reaction zones the methanol feedstock is flowed through a bed containing the ELAPO catalyst. When multiple reaction zones are used, one or more ELAPO catalyst may be used in series to produce the desired product mixture. Instead of a fixed bed, a dynamic bed system, e.g., fluidized or moving, may be used. Such a dynamic system would facilitate any regeneration of the ELAPO catalyst that may be required. If regeneration is required, the ELAPO catalyst can be continuously introduced as a moving bed to a regeneration zone where it can be regenerated by means such as oxidation in an oxygen containing atmosphere to remove carbonaceous materials.